Use hosts. Create a file using any text editor like notepad, gedit, etc. It is simple text with the ip, host and alias, - one address per line, ex:
127.0.0.1 localhost
127.0.1.1 freenas
192.168.0.1 router.yourdomain router
192.168.0.2 freenas.yourdomain freenas
192.168.0.150 freenas2.yourdomain freenas2
# The following lines are desirable for IPv6 capable hosts
::1 ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
The second line should show the name of the computer the hosts file resides, I mean, on freenas2 it should read: 127.0.1.1 freenas2 and so on. You can specify any domain you want, use your imagination, save this file with the name hosts e copy and paste to all your machines.